Methods of increasing the solubility of materials in supercritical carbon dioxide

ABSTRACT

Methods of increasing the solubility of a base in supercritical carbon dioxide include forming a complex of a Lewis acid and the base, and dissolving the complex in supercritical carbon dioxide. The Lewis acid is soluble in supercritical carbon dioxide, and the base is substantially insoluble in supercritical carbon dioxide. Methods for increasing the solubility of water in supercritical carbon dioxide include dissolving an acid or a base in supercritical carbon dioxide to form a solution and dissolving water in the solution. The acid or the base is formulated to interact with water to solubilize the water in the supercritical carbon dioxide. Some compositions include supercritical carbon dioxide, a hydrolysable metallic compound, and at least one of an acid and a base. Some compositions include an alkoxide and at least one of an acid and a base.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/544,532, filed Jul. 9, 2012, pending, which is a divisional of U.S.patent application Ser. No. 11/075,771, filed Mar. 9, 2005, now U.S.Pat. No. 8,241,708, issued Aug. 14, 2012, the disclosure of each ofwhich is hereby incorporated herein in its entirety by this reference.

BACKGROUND

Insulator oxide films, particularly silicon oxide films, haveconventionally been made by methods such as thermal oxidation ofsilicon, physical vapor deposition and chemical vapor deposition, mosttypically chemical vapor deposition. However, chemical vapor depositionrequires high temperatures, e.g., above 300° C., even with the aid of aplasma. Newer, lower temperature techniques, known as Chemical FluidDeposition (CFD), are based on chemical deposition of the oxide filmsfrom a supercritical fluid solution have been developed.

U.S. Pat. No. 4,970,093 describes a method for depositing a film of adesired material on a substrate comprising dissolving at least onereagent in a supercritical fluid comprising at least one solvent. Eitherthe reagent is capable of reacting with, or is a precursor of, acompound capable of reacting with the solvent to form the desiredproduct, or at least one additional reagent is included in thesupercritical solution and is capable of reacting with, or is aprecursor of, a compound capable of reacting with the first reagent orwith a compound derived from the first reagent to form the desiredmaterial. The supercritical solution is expanded to produce a vapor oraerosol and a chemical reaction is induced in the vapor or aerosol sothat a film of the desired material resulting from the chemical reactionis deposited on the substrate surface. In an alternate embodiment, thesupercritical solution containing at least one reagent is expanded toproduce a vapor or aerosol which is then mixed with a gas containing atleast one additional reagent. A chemical reaction is induced in theresulting mixture so that a film of the desired material is deposited.

U.S. Pat. No. 5,789,027 describes methods for depositing a film ofmaterial on the surface of a substrate by i) dissolving a precursor ofthe material into a supercritical or near-supercritical solvent to forma supercritical or near-supercritical solution; ii) exposing thesubstrate to the solution, under conditions at which the precursor isstable in the solution; and iii) mixing a reaction reagent into thesolution under conditions that initiate a chemical reaction involvingthe precursor, thereby depositing the material onto the solid substrate,while maintaining supercritical or near-supercritical conditions. Theinvention also includes similar methods for depositing materialparticles into porous solids, and films of materials on substrates orporous solids having material particles deposited in them.

U.S. Pat. No. 6,541,278 describes a semiconductor substrate that isplaced within a housing. By supplying organometallic complexes andcarbon dioxide in a supercritical state into the housing, a BST thinfilm is formed on a platinum thin film, while at the same time, carboncompounds, which are produced when the BST thin film is formed areremoved. The solubility of carbon compounds in the supercritical carbondioxide is very high, and yet the viscosity of the supercritical carbondioxide is low. Accordingly, the carbon compounds are removableefficiently from the BST thin film. An oxide or nitride film may also beformed by performing oxidation or nitriding at a low temperature usingwater in a supercritical or subcritical state, for example.

U.S. Pat. No. 6,716,663 describes a method wherein a semiconductorsubstrate is placed within a housing. By supplying organometalliccomplexes and carbon dioxide in a supercritical state into the housing,a BST thin film is formed on a platinum thin film, while at the sametime, carbon compounds, which are produced when the BST thin film isformed, are removed. The solubility of carbon compounds in thesupercritical carbon dioxide is very high, and yet the viscosity of thesupercritical carbon dioxide is low. Accordingly, the carbon compoundsare removable efficiently from the BST thin film. An oxide or nitridefilm may also be formed by performing oxidation or nitriding at a lowtemperature using water in a supercritical or subcritical state, forexample.

Although these methods of chemical deposition form supercritical fluidsolutions provide advantages over conventional deposition techniques,they can still be improved. In particular, faster reaction/depositionrates are desired. Also, providing a broader array of precursors andreagents would also be advantageous.

BRIEF SUMMARY OF THE INVENTION

A hallmark of the present invention is the rapid deposition of oxideformations via acid or base catalyzed CFD processes.

In one embodiment, the invention is a method for forming an insulatingstructure, the method comprising hydrolyzing an alkoxide in asupercritical fluid in the presence of an acid catalyst or a basecatalyst such that an insulating oxide material is deposited from thesupercritical fluid to form the insulating structure.

Another embodiment of the invention is a composition comprising asolution of an alkoxide and either an acid or a base in supercriticalcarbon dioxide.

A further embodiment of the invention is a method of forming a materialhaving a high dielectric content, the method comprising the steps offorming a solution of a hydrolysable alkoxide and a catalyst, thecatalyst comprising an acid or a base, in supercritical carbon dioxide;and, reacting the hydrolysable alkoxide with water to deposit an oxidehaving a dielectric constant at least about 10.

Another embodiment of the invention is a method of producing aninsulating film, the method comprising forming a solution of ahydrolysable alkoxide and a catalyst, the catalyst comprising an acid ora base, in supercritical carbon dioxide; contacting a substrate with thesupercritical carbon dioxide solution; and, reacting the hydrolysablealkoxide with water to deposit a film of an oxide having a dielectricconstant at least equal to silicon dioxide.

Yet another embodiment is a method for producing fine structures of aninsulating material, the method comprising forming a solution of ahydrolysable alkoxide and a catalyst, the catalyst comprising an acid ora base, in supercritical carbon dioxide; contacting a substrate with thesupercritical carbon dioxide solution, wherein the substrate comprisesstructures having high aspect ratios of at least 5; and, reacting thehydrolysable alkoxide with water to deposit an oxide having a dielectricconstant at least equal to silicon dioxide, wherein the oxide fills thehigh aspect ratio structures.

In a further embodiment, the invention is a method of increasing thesolubility of acids in supercritical carbon dioxide, the methodcomprising combining supercritical carbon dioxide, a Lewis base that issoluble in supercritical carbon dioxide, and an acid that issubstantially insoluble in supercritical carbon dioxide such that theLewis base and the acid form a complex that is soluble in supercriticalcarbon dioxide.

Another embodiment is a method of increasing the solubility of bases insupercritical carbon dioxide, the method comprising combiningsupercritical carbon dioxide, a Lewis acid that is soluble insupercritical carbon dioxide, and a base that is substantially insolublein supercritical carbon dioxide such that the Lewis acid and the baseform a complex that is soluble in supercritical carbon dioxide.

Yet another embodiment is a method for increasing the solubility ofwater in supercritical carbon dioxide, the method comprising combiningsupercritical carbon dioxide with an acid or base, wherein the acid orbase is soluble, or solubilizable, in supercritical carbon dioxide andthe acid or base interacts with water to solubilize the water in thesupercritical carbon dioxide.

Still yet another embodiment is a method of forming a material having alow dielectric content, the method comprising the steps of forming asolution of a hydrolysable alkoxide and a catalyst, the catalystcomprising an acid or a base, in supercritical carbon dioxide, andreacting the hydrolysable alkoxide with water to deposit an oxide havinga dielectric constant less than about 3.

Various other features, objects and advantages of the present inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, references made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

FIGS. 1A-1D show SEM images of a silicon dioxide film formed by a methodof this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is an improved method of conducting chemical reactions insupercritical, or near supercritical, carbon dioxide (SCD). In onepreferred embodiment, the invention is a method for producing metal orsemi-metal oxide deposits by hydrolysis of at least one hydrolysableprecursor in supercritical carbon dioxide (SCD). Specifically, thehydrolysis reaction can be catalyzed by the presence of either an acidor a base.

The hydrolysable precursor is a typically a hydrolysable metalliccompound. As used herein, the terms “metal” and “metallic” are to beconstrued broadly to encompass metals, the semi-metals (also known asmetalloids) and phosphorus. The semi-metals are typically considered tobe boron, silicon, germanium, arsenic, antimony, tellurium, andpolonium.

The hydrolysable metallic compound precursor must be soluble orpartially soluble in supercritical carbon dioxide (SCD). Unlike a normalfluid solvent, SCD has virtually no surface tension. As such, SCD isfreely miscible with all gases because of the mutual lack of surfacetension. Therefore, the terms “solubility” and “soluble” are used in thebroadest sense to mean the ability or tendency of one substance to blenduniformly with another and the term “solution” is used to designate bothtrue solutions (i.e., solids dissolved in a solvent) and uniformmixtures of miscible fluids. The SCD may include one or more co-solventssuch as an alcohol (e.g., methanol, ethanol, etc.) or other semi-polarsolvent (e.g., acetone) added to further aid in dissolution of the metalalkoxide, metal complex or salt. Additionally, this method could beapplicable to reverse micelle structures that contain a CO₂ immisciblesolvent that is the carrier for one or more of the reactants. Sometypical surfactants for a reverse micelle in SCD arebis-(2-ethylhexyl)sulfosuccinate (AOT), Zonyl FSJ (contains one or morefluoroalkylphosphate ester salt), and poly(1,1,-dihydroperfluoro octylacrylate)-b-poly (ethylene oxide) and others in review article: Helen M.Woods, Marta M. C. G. Silva, Cécile Nouvel, Kenin M. Shakesheff andStven M. Howdle, Materials processing in supercritical carbon dioxide:surfactants, polymers and biomaterials, J. Mater. Chem., 2004, 14 (11),1663-1678.

Generally, the hydrolysable metallic compounds known from the field ofSol-Gel chemistry should be appropriate for use in this inventive methodunder the right processing conditions. Examples of such compounds are:

1) Metal alkoxide with the structure M(OR), such as ethoxides (OEt),propoxides (OPr), butoxides (OBu), etc., and associated oligomersspecies [M(OR)_(n)]_(m), where M is at least one metal atom, R is anyalkyl group and may be the same or different each occurrence, and m andn are constants that are determined as needed to balance the electroniccharge. Preferably, M is at least one of silicon, boron, hafnium,aluminum, phosphorus, zirconium, titanium, barium, lanthanum, oryttrium. Typically, R is a methyl, ethyl, propyl, or butyl group. Anon-limiting list of suitable metallic alkoxides includes silicon tetraalkoxy compounds (such as tetraethyl orthosilicate (TEOS),tetramethoxysilane (TMOS), tetrapropoxysilane (TPOS), andtetrabutyloxysilane (TBOS)), hafnium tert-butoxide, aluminum ethoxideand aluminum isopropoxide. These and other metallic alkoxides arecommercially available, such as from Gelest, Inc. More than one metallicalkoxide precursor may be used when a complex oxide, e.g., BST, is to bedeposited. M-O-M linkages can exist in these materials, as well.Included are reaction products of metal alkoxides with organic hydroxycompounds such as alcohols, silanols R₃SiOH, glycols OH(CH₂)_(n)OH,carboxylic and hydroxycarboxylic acids, hydroxyl surfactants etc.

2) Metal carboxylates M(O₂COR)_(n), and carboxylate oligomers andpolymers [M(O₂CR)_(n)]_(m), as well as hydrates thereof, where M is atleast one metal atom, R is any alkyl group and may be the same ordifferent each occurrence, and m (m stands for the degree of associationor molecular complexity or nuclearity) and n are constants that aredetermined as needed to balance the electronic charge.

3) Metal β-diketonates [M(RCOCHCOR′)_(n)] and oligomeric and polymericmaterials [M(RCOCHCOR′)_(n)], as well as adductsM(β-diketonates)_(n)L_(x) where M is at least one metal atom, R and Ŕare any alkyl group and may be the same or different each occurrence, nis a constant determined as needed to balance the electronic charge, andL usually has a nitrogen or oxygen donor sites such as water, alcohols,ethers, amines, etc.

4) Metal alkoxide derived heteroleptic species (i.e., species withdifferent types of ligands) such as M(OR)_(n-x)Z_(x) (Z=β-diketonates orO₂CR), where M is at least one metal atom, R is any alkyl group and maybe the same or different each occurrence, and m and x are constants thatare determined as needed to balance the electronic charge.

5) ORganically MOdified SILanes (ORMOSILS) of general formula(RO)_(4-x)SiZ_(x) where R is any alkyl group, Z is another functional(e.g., acrylate, epoxide, vinyl, etc.) or non-functional alkyl groupforming a stable Si—C bond, and x is a constant chosen to balanceelectronic charge.

6) Heterometallic precursors (M_(x)M_(y)′, M,My′M_(z)″) with such formsas, but not limited to M_(x)M′_(y)(OR)_(n), where M, M′ and M″ aredifferent metal atoms, R is any alkyl group and may be the same ordifferent each occurrence, and n, x, y, and z are constants that aredetermined as needed to balance the electronic charge.

7) Metal salts, halides MX_(n), chlorates, hypochlorites, nitrates,nitrites, phosphates, phosphites, sulfates, sulfites, etc., where M is ametal atom, X is a halide atom and n is a constant determined as neededto balance the electronic charge.

Non-hydrolytic condensation reactions are also possible with theseSol-Gel materials. Building-up of the M-O-M network can also be achievedby condensation reactions between species with different ligands. Metalalkoxides and carboxylates (elimination of ester, equation 1), metalhalides MX_(n) and alkoxides (formation of alkylhalide—equation 2) orelimination of dialkylether (equation 3) as the source of the oxo ligandare examples.

M(OR)_(n)+M′(O₂CR′)_(n)→(OR)_(n−1)M-O-M′(O₂CR′)_(n−1)+RCO₂R  (1)

M(OR)_(n)+M′X_(b)→(OR)_(n−1)M-O-M′X_(n−1)+RX  (2)

M[OSi(OR)₃]n→MO_(n/2)+SiO₂+R₂O under applied heat  (3)

Metal alkoxides can also be used as precursors of non-oxide materials.For instance, fluorinated alkoxides M(OR_(f))_(n) (R_(f)=CH(CF₃)₂, C₆F₅,. . . ) can decompose upon heating to give the base metal. Metalfluorides may result from these precursors depending on thermaltreatment. The reactivity of the M-OR bond also provides ascention tophosphatessulfides or oxysulfides materials.

The hydrolysable metallic alkoxide precursors are selected so that theyyield the desired metallic oxide material. The metallic oxide materialsmay have high k values (dielectric constant), baseline values, or low kvalues. The high k value materials deposited by the hydrolysis reactionshave k values at least equal to about 10. Typical of such high k valuematerials are typically oxides, such as, for example, Ba—Sr—Ti—O (BST),Pb—Zr—Ti—O (PZT), and certain low atomic number metal oxides or mixedmetal oxides, such as titanium oxide, hafnium oxide, zirconium oxide,aluminum oxide or hafnium-aluminum oxide. Silicon dioxide is generallyconsidered the baseline material having a k value of around 4. Otherbaseline materials include boron phosphosilicate glass (BPSG) andphosphosilicate glass (PSG). Low k value materials (k less than about 3)can be derived from these materials by incorporating fluorine and/orcarbon and/or porosity. Other low k value materials possible by thisinvention are hybrid inorganic-organic glasses that incorporatemetal-organic bonds into the material. Representative of such hybridglasses are organically modified silicate (Ormosil), organicallymodified ceramic (Ormocer), and silicon silsesquioxane materials.

The catalysts are acids or bases that are either soluble insupercritical carbon dioxide or are soluble when part of a Lewisacid-Lewis base complex. Suitable acids include organic acids, such asacetic acid, formic acid, and citric acid, as well as inorganic acidssuch as hydrofluoric acid (gaseous at the critical temperature of SCD),hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid. Manyorganic acids, and hydrofluoric acid, are soluble in supercriticalcarbon dioxide. Likewise, chlorine and bromine are gaseous at thecritical temperature of SCD and form acids in contact with water). Incontrast, many inorganic acids, especially strong inorganic acids, arenot normally soluble in supercritical carbon dioxide. Such SCD-insolubleacids can form SCD-soluble complexes with SCD-soluble Lewis Bases. Aparticularly useful Lewis Base for forming these SCD-soluble complexesis tributyl phosphate. Tributyl phosphate is highly soluble in SCD andthe inventors believe that the phosphate group can attach to acids, suchas nitric acid or HCl, to increase the solubility of the acid by ordersof magnitude. Suitable bases include ammonia, organic amines, pyridineor substituted pyridine, and fluoroamines. Strong inorganic bases, suchas hydroxides, e.g., KOH or NaOH, can be used if they are solubilized bycomplexing with a Lewis acid that is soluble in supercritical carbondioxide.

Generally, the hydrolysis reactions are limited by the low solubility ofwater in supercritical carbon dioxide. The scarcity of available waterdue to the low SCD-solubility of water is believed to be a major causeof the relatively slow reaction rates seen in earlier processes that didnot use the current catalysts. For example, metal alkoxides arewell-known to be moisture sensitive. Indeed, metal alkoxides willtypically undergo hydrolysis slowly at room temperature and would beexpected to rapidly hydrolyze at 100° C., even in the absence of acatalyst, if water was readily available.

In contrast to the previous art, in this method, the SCD-soluble acids,bases and/or acid/base-complexed catalysts interact with watermolecules, so that the SCD-soluble catalysts work as carriers for waterdelivery in supercritical CO₂. This interaction greatly increases theavailability of water for the hydrolysis reaction, which results in thedesired increase in the hydrolysis reaction rate. For example, ammoniaappears to have at least a one-to-one molecular interaction with waterso that, on average, each dissolved ammonia molecule carries at leastone water molecule.

The new acid or base catalyzed oxide deposition process in supercriticalfluid is carried out in a high-pressure system with CO₂ pressure atleast at the critical pressure of about 73 atm, typically greater than80 atm. The concentrations of the precursors (alkoxides) and waterdissolved in the supercritical fluid phase are usually high (severalhundred tons or more) and consequently result in high deposition ratesin relatively low temperatures. Preferably the reaction temperature isno more than about 150° C., more preferably no more than about 100° C.

When using the process of this invention, the deposition rate isgenerally fast, in the order of several hundred angstroms per minute.The oxide films formed by this method show good morphology and strongadhesion to silicon or other substrate surfaces. This method also allowsdeposition of oxides in fine structures of silicon wafers with highaspect ratios. The high diffusivity and low viscosity of supercriticalcarbon dioxide enables oxide deposition in small areas and finestructures with high aspect ratios. FIG. 1 shows SEM images of silicondioxide films formed on a silicon wafer and also deposited in the smallstructures (100 nm wide and 500 nm deep trenches). As shown in FIG. 1,the silicon dioxide films are basically free of visible voids accordingto the SEM micrographs.

Although the oxide films produced by this method are typically free oflarge voids, the films are porous as indicated by the density of thedeposited material. However, due to the lack of surface tension in SCD,the drying occurs without contractional forces from the liquid. As aresult, the deposit material does not display “mud-cracking” typical ofthe drying of a normal fluid solvent. Generally, the oxide films formedby base catalyzed reactions are denser than the oxide films formed byacid catalyzed reactions. The densities of the oxide layers formed in bythis inventive process are believed to be greater than 50% of thedensity of dense SiO₂ (2.2 g/cm³).

Representative examples of acid or base catalyzed oxide formationreactions are described as follows:

SiO₂ Film Formation

When acetic acid is used as the catalyst, a smooth silicon dioxide filmwith reasonable thickness can be formed in supercritical CO₂ attemperatures above 100° C. The deposition reaction actually starts atroom temperature but produces good quality thick films at 100° C. In theabsence of acetic acid, only uneven and thin silicon dioxide films(10-20 nm) can be formed. Addition of acetic acid makes the resultingsilicon dioxide films uniform and thick. The thickness of the silicondioxide films formed by reaction (1) can be up to 500 nm in the presenceof 19 mole % to 95 mole % of acetic acid relative to TEOS. The acidcatalytic reaction probably involves proton coordination to the oxygenatoms of TEOS molecule that facilitates the hydrolysis.

SiO₂ Film Formation

Alkoxide: tetraethyl orthosilicate (TEOS); Base catalyst: NH₃

HfO₂ Deposition

Alkoxide: Hafnium tert-butoxide; Base catalyst: NH₃

Al₂O₃ Deposition

Alkoxide: (a) Aluminum ethoxide and (b) Aluminum isopropoxide; Basecatalyst: NH₃

In compliance with the statute, the invention has been described inlanguage more or less specific as to chemical, structural and methodicalfeatures. It is to be understood, however, that the invention is notlimited to the specific features shown and described, since the meansherein disclosed comprise preferred embodiments of putting the inventioninto effect. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that changesmay be made without departing from the spirit and scope of the presentinvention. The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of increasing the solubility of a base in supercriticalcarbon dioxide, comprising: forming a complex of a Lewis acid and thebase, wherein the Lewis acid is soluble in supercritical carbon dioxide,and the base is substantially insoluble in supercritical carbon dioxide;and dissolving the complex in supercritical carbon dioxide.
 2. Themethod of claim 1, wherein forming a complex of a Lewis acid and thebase comprises reacting the Lewis acid with a base selected from thegroup consisting of an organic amine, a substituted pyridine, afluoroamine, potassium hydroxide, and sodium hydroxide.
 3. A method forincreasing the solubility of water in supercritical carbon dioxide,comprising: dissolving an acid or a base in supercritical carbon dioxideto form a solution, wherein the acid or the base is formulated tointeract with water to solubilize the water in the supercritical carbondioxide; and dissolving water in the solution.
 4. The method of claim 3,wherein dissolving an acid or a base in supercritical carbon dioxidecomprises dissolving an acid complexed with tributyl phosphate in thesupercritical carbon dioxide.
 5. The method of claim 3, whereindissolving an acid or a base in supercritical carbon dioxide comprisesdissolving an acid selected from the group consisting of acetic acid,formic acid, citric acid, hydrofluoric acid, hydrochloric acid, nitricacid, sulfuric acid, and phosphoric acid.
 6. The method of claim 3,wherein dissolving an acid or a base in supercritical carbon dioxidecomprises dissolving acetic acid in the supercritical carbon dioxide. 7.The method of claim 3, wherein dissolving an acid or a base insupercritical carbon dioxide comprises dissolving a base selected fromthe group consisting of ammonia, an organic amine, pyridine, asubstituted pyridine, a fluoroamine, and a hydroxide.
 8. The method ofclaim 3, wherein dissolving an acid or a base in supercritical carbondioxide comprises dissolving ammonia in the supercritical carbondioxide.
 9. The method of claim 3, further comprising combining thesolution with a hydrolyzable alkoxide.
 10. The method of claim 3,wherein dissolving an acid or a base in supercritical carbon dioxide toform a solution comprises forming a complex comprising at least one of aLewis acid and a Lewis base.
 11. The method of claim 3, whereindissolving an acid or a base in supercritical carbon dioxide to form asolution comprises dissolving at least one material in the supercriticalcarbon dioxide, wherein the at least one material alone is insoluble insupercritical carbon dioxide.
 12. The method of claim 1, wherein forminga complex of a Lewis acid and the base comprises forming a complex ofthe base and the Lewis acid selected from the group consisting of aceticacid, formic acid, citric acid, and combinations thereof.
 13. The methodof claim 1, further comprising maintaining a temperature of thesupercritical carbon dioxide at less than about 150° C.
 14. A methodcomprising: combining supercritical carbon dioxide, a base that issoluble in the supercritical carbon dioxide, and an acid that issubstantially insoluble in the supercritical carbon dioxide to form asupercritical carbon dioxide solution comprising a complex of the baseand the acid, the complex soluble in the supercritical carbon dioxide;combining the supercritical carbon dioxide solution with a hydrolyzablealkoxide; and reacting the hydrolyzable alkoxide with water to form asolid material.
 15. The method of claim 14, wherein reacting thehydrolyzable alkoxide with water to form a solid material comprisesforming a solid material that is insoluble in the supercritical carbondioxide solution.
 16. The method of claim 14, wherein forming the solidmaterial comprises forming a material having a dielectric constant ofless than about
 3. 17. The method of claim 14, wherein combiningsupercritical carbon dioxide, a base, and an acid comprises combiningsupercritical carbon dioxide, a Lewis base, and an acid.
 18. The methodof claim 14, further comprising dissolving water in the supercriticalcarbon dioxide solution.
 19. The method of claim 18, wherein dissolvingwater in the supercritical carbon dioxide solution comprises dissolvingwater in the supercritical carbon dioxide solution at a higherconcentration than a solubility limit of water in supercritical carbondioxide alone.
 20. The method of claim 14, further comprisingmaintaining a temperature of the supercritical carbon dioxide solutionat less than about 150° C.